FAMILIAL SUPERNUMERARY MARKER CHROMOSOME EVOLUTION THROUGH 3 GENERATIONS

A mosaic chromosome complement, 36.XY/47.XY.+r(15), was detected at pr enatal diagnosis, Family studies showed the mother and one of her two children to have a bisatellited supernumerary marker chromosome (SMC) in all lymphocytes examined. The maternal grandfather also shove-ed a bisatellited SMC, but in only 2 per cent of his lymphocytes. NOR, DA/D API, and chromosome 15 centromere and short arm-specific probes confir med the identify of the bisatellited SMC and of ring SMC as derived fr om chromosome 15. An apparently normal male was born at full term. At age 1 year, the baby continues to have normal growth and development. The bisatellited 15 likely originated by somatic mutation in the grand father (2 per cent cells), was transmitted unchanged to the daughter a nd grandson (germline transmission, no mosaicism), and then evolved by excising the satellites and forming a ring SMC in the index case. Pro gressive changes in the frequency and subsequent changes in the struct ure of this SMC illustrate the unusual characteristics of chromosome 1 5. (C) 1998 John Wiley & Sons. Ltd.